AbstractIn modem agriculture, agrochemicals are frequently used for crop protection. Even though 20-40% of the world's crop production is lost annually because of the effects of weeds, pests and disease. Crop losses to pests would be doubled if existing practices of pesticide usage were abandoned. Under normal agricultural practices, tremendous amounts of agrochemical are used each year to boost up crop production. Farmers use such chemical for economic reasons but , they are imposing environmental pollution that may disturb the ecosystem. This heavy load of pesticide usage on crops and soil is of particular concern. The soil burden resulting from the repeated long-term applications of pesticide for protection of cotton is of special concern in our country because more than 80 % pesticides are used on this crop . Such extensive pesticide treatments may suppress soil micro flora or their functions and hence affect soil properties. Chemical and biological methods like bacterial and fungal populations, respiration, Fe (III) reduction, nitrification rates (measuring nitrate formation from substrate such as (NH4)2 S04) and soil enzymes (dehydrogenase and arginine deaminase) were used to assess the effect of pesticide applications in cotton agroecosystem under field condition

Total nitrogen and available phosphorus were not affected by the application of pesticides; however, organic matter was increased after their application. Soil bacterial populations were inhibited by endosulfan alone or with dimethoate. Monocrotophos and a mixture of bifenthrin + acetamiprid increased the bacterial population. Fungal population was increased with endosulfan + dimethoate, profenophos + cypermethrin, fenpropathrin, methamidophos, monocrotophos, befenthrin + acetamiprid, profenophos with ethion or with a mixture of carbosulfan and chlorpyrifos while endosulfan decreased it. Overall inhibition was observed in total number of bacteria and stimulation in fungal population at 2 days after all pesticides application in different seasons that continued to the time of harvest of crop but total number of microbes recovered to the normal level later on as were at sowing time. Successive application of pesticides caused only slight and short-lived side effects on the soil respiration and microbial biomass and these usually disappeared before the next treatment was carried out. Samples collected after all pesticides application and at harvest stage in each season showed same basal respiration as well as microbial biomass. Methamidophos, profenophos alongwith diafenthiuron or with alphamethrin, bifenthrin with acetamiprid or with ethion inhibited while endosulfan alone or with bifenthrin and a mixture of bifenthrin + carbosulfan + chlorpyriphos stimulated the Fe (III) reduction capacity of soil. All other pesticide have no effect on Fe (III) reduction capacity of soil. Mixed results were observed for the effect of pesticides application on the soil nitrification. Endosulfan alongwith dimethoate stimulated but with bifenthrin it inhibited the nitrification. Profenophos alongwith cypermethrin inhibited while with alphamethrin it was stimulated. Endosulfan, imidacloprid, a mixture of chlorpyriphos + trolomethrin + acetamiprid and cyhalothrin + profenophos + diafenthiuron enhanced while othofonprix decreased the nitrification after their application in soil Samples collected before and after pesticides application for each cropping season showed hat the effects were short lived and nitrification was same as at the sowing time in each season. The applied insecticides have no significant effect on enzyme activities (dehydrogenase and arginine deaminase) in this study.

Data from the experiments on bound residues showed that pesticides became bound to soil and the amount of total bound residues increased with time. T results also indicated that with repeated application of other pesticides to the soil, the amount of soil bound residues- of pesticides decreased. The estimated time required for loss of 50%(over all half-life) of 14C- methamidophos was 170 days while it was 124 days for l4C-methaidophos + cold pesticides. Half-lives calculated for the initial rapid phase were 86 and 77 for 14C-methamidophos and 14C- methamidophos + cold pesticides respectively. Small quantity of pesticides moves down into the deeper soil zones, which may create a risk for ground water contamination. From the study, it is evident that the applied insecticides have no significant effect on the bioactivities of soil micro flora. Inhibition or stimulation of micro flora or their functions observed after pesticides application could be due to the effect of the individual insecticide or it could be due to synergistic or antagonistic. effect of the subsequent insecticide treatment. At the end of each cropping season, these activities were similar in all the fields indicating that the pesticides had only transient effect and recovery of microorganisms and their activities were observed after the treatment effects were over.